Scientists and engineers often employ geophysical surveys for exploration and engineering projects. Geophysical surveys can provide information about underground structures, including formation boundaries, rock types, and the presence or absence of fluid reservoirs. Such information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Oil companies in particular often invest in extensive seismic and electromagnetic surveys to select sites for exploratory oil wells.
Geophysical surveys can be performed on land or in water using active seismic sources such as air guns, vibrator units, or explosives, to generate seismic waves, and further using receivers such as hydrophones or geophones, to detect reflections of such waves from subsurface structures. The process is repeated with many different source positions and optionally with different receiver positions. The arrangement of sources and receivers may be customized to achieve adequate coverage of the region of interest while facilitating processing of the acquired data.
The acquired seismic data is recorded and processed to provide a seismic image that may be used to identify subterranean features of interest. Seismic imaging methods include ray-based methods (such as Kirchhoff migration), wave-equation based methods (such as one-way imaging or two-way, reverse-time migration, imaging), and beam propagation-based methods (such as Gaussian beam migration). In general, ray-based seismic imaging methods are relatively computationally cheap but tend to have difficulty imaging complex subsurface features while wave-equation methods are more accurate, but computationally expensive. (The computational burden associated with reverse time migration makes it generally impractical for use in iterative imaging or velocity inversion.) Beam propagation-based methods tend to fall in between ray-based methods and wave-equation based methods in computational cost and accuracy.
Regardless of the imaging method employed, measurement noise can obscure the subsurface structures, particularly when wide-azimuth all-shots-based inversion is performed. To combat noise accumulation resulting from inclusion of a large number of shots having little or no information about a particular region of interest, the analyst may employ “target-oriented imaging”, a inversion method based on only those shot traces that are suitably selected or weighted by their relevance to the target region rather than including the full set of data. While this method may dramatically reduce the volume of seismic data being inverted and thereby reduce the computational burden, existing techniques for performing the selection or weighting may themselves be computationally intensive.